The invention generally relates to a switching device with an actuator element. Preferably, it includes a shape memory alloy, into which a predetermined shape has been impressed at an annealing temperature and which is connected to a movable contact part of a switching contact. Preferably, a device is included for heating up the actuator element above a temperature level bringing about an opening of the switching contact on the basis of a change in shape of the actuator element.
A known switching device is disclosed by the publication xe2x80x9cEngineering Aspects of Shape Memory Alloysxe2x80x9d, published by Butterworth-Heinemann, London (GB) 1990, pages 330 to 337.
Standard circuit-breakers, as are known for example as Siemens circuit-breaker standard range 5SX2/5SX4, have in their current path a magnetically quick-tripping short-circuiting switching contact. This switching contact additionally has a delayed trip for current limitation, in that it can also be thermally opened. For this purpose, a bimetallic strip which is connected to a movable contact part of the switching contact and is indirectly heated up when there is an overload is generally integrated into the current path. This heating-up is accompanied by a curving of the bimetallic strip, which leads to an opening of the switching contact. When the heating ceases, the bimetallic strip returns to its extended shape, closing the switching contact.
It is known from the publication mentioned at the beginning xe2x80x9cEngineering Aspects of Shape Memory Alloysxe2x80x9d to replace such bimetallic strips by strip-shaped actuator elements consisting of a shape memory alloy. Actuator elements of this type must therefore undergo corresponding curving effects when they are heated up. It is therefore considered necessary to impress a correspondingly curved shape into these elements at relatively high temperatures of, for example, 600 to 850xc2x0 C. After triggering the shape memory effect at an elevated temperature, for example over 200xc2x0 C., the transition into the impressed curved shape then takes place. At lower temperatures, in an operating state in which the switching function is not triggered, between approximately room temperature and approximately 200xc2x0 C., an extended shape of the actuator element is ensured by use of an additional spring element. Thus, a movable contact part of a switching contact mechanically connected indirectly to the actuator element then rests against a fixed contact part.
The production of a corresponding actuator element is relatively cost-intensive, however, because of the annealing at high temperature for the impressing of the curved shape.
It is therefore an object of an embodiment of the present invention to design the switching device in such a way that lower-cost actuator elements including a shape memory alloy can be used.
An object can be achieved according to an embodiment of the invention by providing an actuator element into which an at least largely extended shape has been impressed. This can be done at the annealing temperature. Further, it can include a curved shape in the operating state in which the switching function is not triggered, and which rests between its one end, which is held fixed; and at its other end, which is facing the movable contact part, on a deflecting element with frictional engagement in such a way that the deflecting element exerts on the concave inner side of the actuator element a counterforce partially counteracting the curving of the latter.
Advantages associated with this configuration of the switching device can be seen on the one hand in that a low-cost annealing of the actuator element in an at least largely extended, i.e. straight, shape (with the inclusion of slight deviations from this) is made possible, in particular in the rolled state of a corresponding metal sheet. The consequence of this is that the actuator element can assume a curved shape in the operating state at low temperature.
The curving of the actuator element can in this case be achieved in various ways, including but not limited to: the actuator element having what is known as a 2-way effect on account of corresponding preparational conditions, i.e. two different shapes (curved and extended) have been impressed into it in a way known per se for the two different temperature ranges (of the operating state and triggering state), so that the element curves of its own accord at the lower temperature; and in the case of actuator elements with what is known as a 1-way effect, the curved starting position can be ensured by a special (external) restoring spring. The force to be expended for this purpose is relatively low on account of the material.
In the case of these types, however, without the use of a deflecting element according to an embodiment of the invention, the electrical and mechanical connection of the actuator element at its fixed end to a part of the switching device can be subjected to loading on account of a relatively high lever effect during its thermally induced change in shape. This can occur since the customary alloys of actuator elements with shape memory properties tend on account of their general intermetallic crystalline structure toward brittle mechanical behavior, which specifically in the connecting technique required at the end mentioned, for example by welding or clamping, can have potentially disadvantageous effects on the quality of the corresponding contact point.
However, corresponding potential disadvantages can at least largely be eliminated by the use, according to an embodiment of the invention, of the deflecting element. This can occur since this deflecting element is arranged such that it is fixed in such a way that a force which attempts to bend the actuator element back in the direction of its extended shape is exerted on the actuator element that is in fact curving at the operating temperature. This counterforce is then discontinued when the actuator element is heated, in that the actuator element goes over at least largely into its extended shape. This produces a major advantage of a mechanical relief of the actuator element in a mechanical connecting region (clamping D point) of its fixed end during frequent movements for opening and closing the switching contact.
Since shape memory materials are generally not as low in cost as bimetal, it is generally attempted to reduce the use of material for corresponding circuit-breaker device with overcurrent trip by actuator elements including shape memory material. Problems can be encountered here when using corresponding actuator elements such as those in the prior art according to the cited publication xe2x80x9cEngineering Aspects of Shape Memory Alloysxe2x80x9d with regard to the mechanical stability at the clamping point if the strip-shaped actuator elements are designed to be too narrow and too thin. This can occur since lever effects can cause undesired deformations to occur at these elements, which can result in the failure of the switching contact. The partial bending-straight, according to an embodiment of the invention, of the actuator element by use of the deflecting element significantly counteracts this problem. This is so since the resting effect brings about the mentioned significant relief of the mechanical connection at the fixed end.
A further advantage of the use of a corresponding deflecting element is the way in which it governs the bending-straight of the actuator element. Since the connecting point at the fixed end of the actuator element represents a mechanical weakpoint on account of the lever arm and, although the strip-shaped actuator element would bend straight, the torsional moment at the contact point induces a curving effect, the use of a deflecting element of this type is indeed particularly important.
In addition, the actuator elements which can be used for the switching device are relatively low in cost. This is so because the desired switching behavior can also be achieved with a significant reduction in the volume of the shape memory material, compared with the customary actuator elements, for example according to the publication cited at the beginning.
Other advantageous configurations of the switching device can also emerge according to other embodiments of the invention.
For instance, a restoring spring keeping its actuator element in its curved shape in the operating state may be provided, in particular for the switching device. In this way, relatively low-cost actuator elements including shape memory alloys with what is known as a 1-way effect can be used.
Furthermore, it is advantageous if the actuator element is connected to the movable contact part electrically by using a stranded wire and mechanically by use of a switching linkage. Use of the stranded wire indicates that the mobility of the movable end of the actuator element is virtually unrestricted. The actuator element can consequently be integrated into a current path.